Kinematics of deformation at the southwest corner of the Monument uplift

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Authors Kiven, Charles Wilkinson, 1949-

Publisher The University of Arizona.

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Link to Item http://hdl.handle.net/10150/555078 KINEMATICS OF DEFORMATION AT THE

SOUTHWEST CORNER OF THE MONUMENT UPLIFT

by

Charles Wilkinson KLven

A Thesis Submitted to the Faculty of the

DEPARTMENT OF GEOSCIENCES

In Partial Fulfillment of the Requirements For the Degree of

MASTER OF SCIENCE

In the Graduate College

THE UNIVERSITY OF ARIZONA

1 9 7 6 STATEMENT BY AUTHOR

This thesis has been submitted in partial fulfillment of re­ quirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judg­ ment the proposed use of the material is in the interests of scholar­ ship. In all other instances, however, permission must be obtained from the author.

SIGNED: ( hfiLWA

APPROVAL BY THESIS DIRECTOR

This the Lb has been approved on the date shown below:

A/a

The idea for this study was conceived during the summer of 1974

while I was under contract with the Office of Arid Lands Studies and the

Department of Geosciences, University of Arizona, to investigate the

large-scale fold structures of the portion of the Colorado Plateau tec­

tonic province within the state of Arizona. That regional study was

jointly funded by the National Aeronautics and Space Administration

Grant No. NGL-03-002-313, the Arizona Oil and Gas Conservation Commission

and the Department of Geosciences, University of Arizona. The air photos

used for field mapping were obtained from the Arizona Highway Department.

I would like to extend thanks to a number of diverse groups who

probably do not know how they influenced the inception and outcome of

this study. Some recognition must go to the Organization of Petroleum

Exporting Countries whose oil embargo in late 1973 produced the need for

the type of detailed exploration for potential petroleum reserves which

helped to conceive this study.

On a more personal level I would like to thank the people of the

Navajo Nation, who kindly extended me permission to explore their varied

and beautiful homeland. Mr. Martin link, Head of the Museum and Research

Department of the Navajo Tribal Museum, was very helpful as a liaison be­

tween me and the Navajo Tribal Council and in keeping an eye out for me

during the long summer of 1974• To Messrs. R. Terrence Sudden, Gene

Suemnicht, and Thomas McGarvin, thank you for your unique discussions

iii iv on not only academic matters, but also on such worldly subjects as huge tracts of real estate. Deepest gratitude is extended to all my friends who never could figure out what I was up to but who helped me through the rough times anyway.

Above all others I am most thankful to Dr. George H. Davis, who was the source of the regional study from which this work sprang. His ideas, discussions, and creative criticisms were of incalculable value to me. But valued beyond his ability as a teacher is the bond of mutual respect and friendship which has grown during our association. TABLE OF CONTENTS

Page

LIST OF ILLUSTRATIONS...... vi

HST OF TABLES ...... x

ABSTRACT ...... xi

INTRODUCTION...... 1

Purpose of the S t u d y ...... CM H H Methods ...... Previous Workers ...... Monoclines to Basement Structures Age of the Mbnqclinal Folding...... 10

STRUCTURAL G E O L O G Y ...... 12

Fol d s ...... 12 J o i n t s ...... 24 Faults ...... 30

DISCUSSION ...... 47

Introduction...... 47 Basement Structures Associated with Uplifts ...... 48 Inferred Basement Structures in the Study Area ...... 51 Kinematics of Basement Block Movement...... 55 Deformation in the Sedimentary R o c k s ...... 64 Kinematics of Distributed Shear ...... 70

CONCLUSIONS...... 78

REFERENCES...... 80

v LIST OF ILLUSTRATIONS

Figure Page

1. Structure-Geologic Map of the Southwest Corner of the Monument Uplift ...... in pocket

2. Structure Map of Folds in Phanerozoic Rocks, Colorado Plateau Tectonic Province of Arizona . . . in pocket

3* Schematic Diagram of a Monoclinal Fold Showing the Spatial and Geometric Relationships of the Limbs and Hinges ...... 13

4* Lower-Hemisphere, Equal-Area, Pole-Density Diagram of Poles to Strata that Comprise the Lower Limb of the Organ Rock Monocline and the Upper limb of the Comb Ridge M o n o c l i n e ...... ••... 15

5. Photograph of the Organ Rock Monocline ...... 16

6. Lower-Hemisphere, Equal-Area r Pole-Density Diagram of Poles to Strata in the Middle and Upper limbs of the Organ Rock Mo n o c l i n e ...... 18

7* Lower-Hemisphere, Equal-Area, Pole-Density Diagram of Poles to Strata in the Middle limb of the Cow Springs M o n o c l i n e ...... 20

8. Lower-Hemisphere, Equal-Area, Pole-Density Diagram of Poles to Strata in the Middle limb of the Comb Ridge Monocline...... 23

9. Lower-Hemisphere, Equal-Area, Pole-Density Diagram of Poles to Joints Collected in the Betatakin A r e a ...... 26

10. Lower-Hemisphere, Equal-Area, Pole-Density Diagram of Poles to Joints Measured in Narrow Canyon in the Formations of the Glenn Canyon Group ...... 28

11. Lower-Hemisphere, Equal-Area, Pole-Density Diagram of Poles to Joints from the Middle limb of the Comb Ridge Monocline...... 29

vi vii

LIST OF ILLUSTRATIONS— Continued

Figure Page

12. Lower-Hemisphere, Equal-Area, Pole-Density Diagram of Poles to Joints from the Middle limb of the Organ Rock Monocline...... 31

13• Lower-Hemisphere, Equal-Area, Pole-Density Diagram of Poles to Joints Measured in the Formations of the Glenn Canyon Group Exposed in the Middle limb of the Cow Springs Monocline...... 32

14* Discordant Faults Showing Minor Displacements...... 33

15* Lower-Hemisphere, Equal-Area, Pole-Density Diagram of Poles to Discordant Faults Measured in the Middle limb of the Comb Ridge Monocline ...... 35

16. Lower^Hemisphere, Equal-Area, Pole-Density Diagram of Poles to Discordant Faults in the Middle limb of the Cow Springs Mo n o c l i n e ...... 36

17* Lower-Hemisphere Plot of Poles to Discordant Faults in the Organ Rock M o n o c l i n e ...... 37

18. Oblique-Slip Slickensides and Polish on Discordant Fault Surface in Middle limb of Organ Rock M o n o c l i n e ...... 3$

19. Chatter Marks and Slickensides on Large Block of Kayenta Sandstone from the Middle limb of the Comb Ridge Monocline ...... 39

20. Sketch Showing Full Relationships of Cross-Fractures on Calcite-Bearing Discordant Fault in Kayenta F o r m a t i o n ...... 40

21. Cross-Fractures on Calcite-Bearing Discordant Fault in Kayenta Formation in Middle limb of Comb Ridge M o n o c l i n e ...... 40

22. Gouge-Bearing Concordant Fault in Aeolian Cross- Bedded Navajo Sandstone in the Middle limb of the Cow Springs Monocline ...... 43

23. Slickensides on Concordant Fault Surface in the Navajo Sandstone in the Middle limb of the Comb Ridge Monocline ...... 44 viii

LIST OF ILLUSTRATIONS— Continued

Figure Page

24. Lower-Hemisphere, Equal-Area, Pole-Density Diagram of Poles to Aeolian Cross-Bed Surfaces in the Navajo Sandstone from the Middle limb of the Comb Ridge Monocline ...... 45

25• Lower-Hemisphere, Equal-Area, Pole-DensityDiagram of Poles to Concordant Faults from the Navajo Sandstone from the Middle limb of the Comb Ridge M o n o c l i n e ...... 46

26. Schematic Cross-Section of Basement Structure...... 50

27. Reproduction of a Portion of the Residual Aeromagnetic Map of Arizona (Sauck and Sumner, 1971) ...... 52

28. Reproduction of the Residual Gravity Map of Arizona (West and Sumner, 1 9 7 3 ) ...... 53

29. Structural Relief M a p ...... 56

30. Graph of the Change in Vertical Displacement Versus Angle of a Fau l t ...... 59

31• Block Diagram Showing Geometric Relationships of Basement Blocks ...... 6l

32. Models Showing Relative Periods of Vertical Movement for the Three Basement Blocks. Tsegi Block = (A); Monument Block = (B)j Black Mesa Block = ( C ) ...... 63

33• Photograph of the Hunter’s Point Monocline near Window Rock, Arizona ...... 66

34* Pole-Density Diagram Generated by Rotating Selectively Defined Joint Sets 20° to the East about a North- South Axis ...... 68

35* Graph Showing the Number of Degrees of Change in Dip for a Joint Set Versus the Angle between the Pre­ rotation Strike of the Joint Set and the Axis of Rotation about which the Strata Containing the Joint were Rotated...... 69

36. Lower-Hemisphere Projection of Poles that Represent Joint Maxima Measured in Horizontal (x’s) and Folded Strata (o’s) 71 ix

LIST OF ILLUSTRATIONS— Continued

Figure Page

37• Strain Ellipse Resulting from the Shear Couple Set up by the Draping of the Sedimentary Cover Rocks during Monoclinal Folding ...... 72

3fi>. Schematic Diagram Showing the Inferred Change of Attitude of Pre-folding Joints during Folding, and the Relative Movement of Individual Blocks Bounded by these Surfaces ...... 74

39* Geometric Diagram Showing how the Increase of the Width of the Middle Limb Zone and the Decrease in Dip of that Zone would Result in a Decrease in the Amount of Extensional Strain Higher in the Fold • . 76 LIST OF TABLES

Table Page

1. Stratigraphic Units in the Area of the Kayenta Salient ...... 4

x ABSTRACT

The Kayenta salient is a complex junction of three Laramide(?) monoclines at the southwest corner of the Monument uplift. The geometry of this junction may be explained by unequal components of vertical movement of three large fault-bounded basement blocks. Through analogy to exposed basement structure of other monoclines in the Colorado Pla­ teau, interpretation of geophysical and geologic data, and the determi­ nation of the axes and the sense of rotation of these blocks, the bounding faults are inferred to be high-angle reverse faults.

The vertical movements of the basement blocks resulted in the passive draping of the Phanerozoic rocks over the edges of the blocks, producing an extensional stress regime. However, the large-scale ten-

sional faults that would be expected in such a regime are absent from

the middle limb zones. The extensional stress produced in these rocks was accommodated by flexural flow in the incompetent units and by a brittle form of boudinage in the competent units. During folding a per­ vasive pre-existing fracture pattern was utilized to accommodate exten­

sion in the competent units. Individual blocks defined by these fractures

separated and were displaced less than 1 meter (3.2 feet) in a stair step

fashion; i.e., blocks closer to the lower hinge were down-dropped with

respect to those further from the lower hinge. The utilization of pre­

existing fractures to accommodate stress precluded the need to initiate

large-scale faulting in the middle limb zone.

xi INTRODUCTION

Purpose of the Study

The southwest comer of the Monument uplift is a complex junc­ tion of three Laramide(?) monoclines near the Navajo town of Kayenta,

Arizona. The three folds that form this junction are the Organ Rock,

Cow Springs and Comb Ridge monoclines. The north-trending, east-facing

Organ Rock monocline swings to the southwest near Marsh Pass to become the northeast-trending, southeast-facing Cow Springs monocline (fig. 1, in pocket). In the vicinity of this junction the Comb Ridge monocline trends northeast, faces southeast, and demarcates the southern boundary of the Monument uplift. This segment of the Comb Ridge monocline can be shown to be continuous with the Cow Springs monocline, contrary to most existing geologic and tectonic maps of the area. The distinct branching character of this junction puts constraints on the possible kinematic models that might realistically explain its origin. Therefore, the structural geology of the area was mapped to determine if a kinematic model might be designed for the observed deformation.

Methods

During the summer of 1974> the author was the primary field geol­ ogist, under the direction of Dr. George H. Davis, on a project designed to precisely locate and describe all the large-scale fold structures in

Arizona’s portion of the Colorado Plateau tectonic province. This work was funded jointly by the National Aeronautics and Space Administration

1 2 grant, NGL 03-002-313» the Arizona Oil and Gas Conservation Commission,

The University of Arizona’s Office of Arid Lands Studies and Department of Geosciences. Structural data describing the monoclines were plotted in the field on LANDSAT-1 imagery at a scale of 1:500,000. These data were transferred to a topographic base map of the State of Arizona, also scaled at 1:500,000. To this were added the existing data describing the attitudes and locations of megascopic folds in northern Arizona. A copy of that map is included herein as figure 2, in pocket (Davis and Kiven,

1975)i for it provides a useful regional structural overview from which to assess the geologic framework of the study area. The characteristic morphology of the Kayenta junction is clearly shown on this map.

Following the regional structural mapping, detailed structural analysis of the Kayenta junction was undertaken at a 1:90,000 scale.

Aerial photographs (1:90,000) provided by the Arizona Highway Department were used as a base for mapping. The data collected were transferred to a base map compiled from portions of four U.S.G'.S. topographic sheets and scaled at 1:62,500. The stratigraphy of this area has been described by several previous workers and thus is not described in detail here. The structural data collected in the field were analyzed by computer and plotted on lower-hemisphere, equal-area, pole-density diagrams.

Previous Workers

The area encompassed by this study has been mapped in part or totally by several previous workers. The area is presented on

1:1,000,000 tectonic maps of the Colorado Plateau prepared by Kelley

(1955a) and Kelley and Clinton (i960). The authors ascribe horizontally- directed compressive forces to explain the origin of the monoclinal folds. 3

Doeringsfeld, Amuedo and Ivey (1958) also mapped the area as a portion of a tectonic map of the Black Mesa Basin. Scurlock (196?) prepared a structure map of northern Arizona that included the study area. Works by Harshbarger, Repenning, and Irwin (1957), and Page and Repenning

(1958) present the Mesozoic stratigraphy of the Navajo and Hopi Indian

Reservations; these papers include geologic maps of a region which in­ cludes the Kayenta junction. Cooley, Harshbarger, Akers and Hardt

(1969) examined the regional hydrogeology of the Navajo and Hopi Indian

Reservations in a paper that includes descriptions of the structure and stratigraphy of the study area. Witkind and Thadden (1963) and Beaumont and Dixon (1965) mapped portions of the study area at a scale of

1:62,500, paying special attention to the distribution of uranium and thorium deposits. These latter two papers contain geologic maps with structure contours. The stratigraphy contained in the aforementioned five papers was synthesized and is presented in table 1.

Dynamic Models of the Origin of Monoclines

Dynamic models that have been proposed to explain the origin of

the monoclines of the Colorado Plateau tectonic province can be roughly divided into two categories: l) models emphasizing the dominant role

of a strong component of vertical uplift, with the monoclines produced by the draping of the sedimentary cover over high-angle faults which

bound large crystalline basement blocks, and 2) models emphasizing the

role of strong horizontal compression and thrust faulting of basement

blocks to produce the monoclinal folds in the upper crustal layers. Table 1. Stratigraphic Units in the Area of the Kayenta Salient.

/iunAGEj rFORMATION vurin. i jlvim wilOR vjntwGROUPui ______CHARACTERISTICS Dune sand; unconsoli— Alluvium; horizon- Talus and landslide ’ dated & semiconsolidated, tally stratified silt, deposits; composed of SURFICIAL DEPOSITS windblown sand & silt- sand & gravels of the angular, unsorted 1 sized, quartz, clay & Naha, Tsegi & Jeddito boulder- to clay- minor feldspar particles formations. sized particles. g LL11W11J. VI J. HU. Ujf Dikes and plugs of augite minette; iron, bronze colored biotite, augite and olivine in a groundmass of alkali feldspar; weathers ERUPTIVE ROCKS to a moderate greyish-brown, fresh surface is light grey to greenish-

TERTIARY grey. Yale Yellow, buff, or brown, fine- to medium-grained, subangular to sub­ points rounded, quartz sandstone; locally containing beds of shale and siltstone. Wepo Very fine-grained to medium-grained, grey to tan, friable to well indu­ Formation rated, cross-bedded sandstone and dark brown or purplish-brown claystone, GROUP siltstone and shale; containing coal beds; forms cliffs and steep slopes.

MESAVERDE Toreya Fine- to medium-grained, grey and tan, cross-bedded sandstone; with a sandstone medial silty and sandy marine shale; forms cliffs. Light grey, dark grey, and tan, clay shale, silty shale, and lesser amounts of very fine-grained silty sandstone and silty shale; containing MANGOS SHALE abundant fossils and selenite; weathers to shallow slopes of light and dark grey and tan soil.

UPPER CRETACEOUS ; Grey to yellowish-brown and brown, medium- to coarse-grained, cross- DAKOTA SANDSTONE bedded, irregularly to thick bedded, fossiliferous sandstone; contains a medial carbonaceous shale member. * UUIVVUU. VM. ili-U UJ Westwater Light yellow and moderate greyish-yellow, fine- to coarse-grained, Canyon friable sandstone, and minor reddish-brown to greenish-grey sandy silt- Member stone or shale; forms cliffs. 1 Pale reddish-brown to greyish-pink, cross-bedded, bench-: and slope­ Recapture forming, fine- to medium-grained quartz sandstone; with large amounts 1 Member of coarse-grained, conglomeratic material; interbedded with dark reddish-brown to greenish-grey lenticular mudstone. FORMATION MORRISON Light grey to greyish-brown cross-bedded, intercalated, fine- to Salt Wash coarse-grained sandstone with greenish-grey and dark red brown, l Member lenticular, flat—bedded, fissile mudstone. ______Massive, greenish-grey to yellowish-grey, subangular to rounded, well «, Cow sorted, fine-grained, cross-bedded, calcareous cemented quartz (with »# Springs Sandstone some feldspar) sandstone; contains some silt and clay. Pinches out in Marsh Pass Area. Grey, light reddish-orange, and light tan, poorly sorted, very fine­ o Sumerville- grained to medium-grained silty sandstone; and friable silty sandstone 1 i Entrada § s and sandy siltstone. Covered with surficial deposits throughout study Formation to area. Slope-forming, reddish-orange brown; flat-bedded sandy siltstone; 1 Carmel interbedded with cliff-forming, fine-grained quartz sandstone; concealed Formation in study area. '1— nm■Prwim-i 4-tr O Fine- to medium-grained, reddish-orange to greyish-orange, massive Navajo aeolian cross-bedded sandstone; interbedded with a few thin lenticular a Sandstone beds of cherty limestone or marl. Most conspicuous unit in study area; -forms~tall-vertical" cliffs. _____—------Pale reddish-brown to pale red-purple, subrounded to subangular, fine- Kayenta to medium-grained sandstone; with lesser amounts of clay shale. Forms Sandstone 0 slopes and benches. V4. ULl.U2f ' 1 Moenave-Wingate(?) Reddish-brown, cliff-forming, aeolian cross-bedded, fine- to medium- o Sandstone grained sandstone. Grey green, brown and purple red, variegated claystones, silt stones, CHINLE FORMATION and very fine-grained sandstones and thin limestones; with basal coarse-grained, cross-bedded quartz conglomerate; forms gentle slopes. Thin, evenly bedded dark brown to chocolate brown sandy shale with many MOENKOPI SANDSTONE beds of red brown, ripple marked, medium-grained sandstone.______— ------

Hoskinninni Deep red brown to maroon, streaked with grey, sandy mudstone and

CUTLER Tongue PERMIAN siltstone. FORMATION 5

The concept of differential vertical uplift as a mechanism for the formation of the monoclines was first proposed by Powell (1873),

Dutton (1880) and Gilbert (1877)• They believed that the monoclines were produced by the draping of the Phanerozoic rocks over the edges of large basement blocks that had been uplifted in response to vertically directed forces. This hypothesis was not employed again in a serious manner until Gilkey (1953) examined the structure of the Zuni uplift in

New Mexico, which is bounded on the south and northwest by monoclines.

He compared the fracture pattern of the Zuni uplift to that of an ideal anticline of compressional origin and to. that of an ideal dome produced by vertically oriented forces. Based on the similarity between the frac­ ture pattern of the Zuni uplift and that of a dome, he inferred that the

Zuni uplift and associated monoclines had been produced by vertically directed forces. More specifically, he inferred that the Zuni uplift is the surface expression of several horsts and grabens in the crystalline

basement.

Dushatko (1953) studied the structural geology of the Lucero

uplift on the eastern border of the Colorado Plateau in New Mexico. The

Lucero uplift is bounded on the west side by a west-facing, faulted

monocline with associated gravity-induced folds. like Gilkey, Dushatko

focused on studying the fracture pattern of the Lucero uplift and defin­

ing the stress pattern that had produced it. He recognized a high density

of joints parallel or subparallel to the long axis of the uplift, and ob­

served that the bisectrix of the obtuse angle produced by the intersection

of the fractures is normal to the long axis of the uplift. By interpret­

ing the stress orientations related to the fracture sets, he inferred 6 that the direction of primary extension was normal to the long axis of the uplift. He concluded that the Lucero uplift was produced by verti­ cally oriented forces in the basement that resulted in the differential vertical uplift of a block, over the edges of which the sedimentary cover was draped to produce a monocline.

Thom (1955)» working in the Wyoming tectonic province, analyzed several uplifts and concluded that these had resulted from differential vertical uplift of a wedge-shaped basement block bounded by high-angle reverse faults. He suggested that the uplifts represent a response to a horizontal compression of undetermined magnitude. Prucha, Graham, and

Nickelson (1965) noted that many of the uplifts of the Wyoming province were bounded by faulted monoclines. They concluded that the uplifts were the result of differing components of vertical motion of crystal­ line basement blocks, along high-angle reverse faults, over which the sedimentary rocks were draped to produce monoclines. In contrast, Berg

(1962) suggested that low-angle overthrusting in the basement could ex­ plain the uplifts of the Wyoming province. Mathews, Callahan and Work

(1975) noted that the folds of the Front Ranges of the Central Rocky

Mountains of Colorado, which have the form of tilted basement blocks with the sedimentary rocks draped over the edges, similar to monoclinal folds, were the result of differential vertical uplift.

The second category of dynamic models for the origin of the mono­ clines emphasizes the role of strong horizontal compression. The propo­ nents of horizontal compressional models either state or imply that wholesale shortening of the crust took place during deformation within the Colorado Plateau. Baker (1935) first proposed that the uplifts and 7 associated monoclines were the result of strong east-west compression.

He suggested that the monoclines are the upper-crustal expression of thrust sheets in the basement, analogous to those observed to the west in the thrust belts of Utah and Nevada. One characteristic noted by

Baker was the tendency for the monoclines to "terminate by flattening as they swing westward around the ends of upwarps, a condition that does

not suggest a decrease in the amount of flexing of strata over a normal

fault as the displacement of the fault decreases'* (Baker, 1935» p* 1502).

Shoemaker (1956) further refined this model and suggested that the up­

lifts and associated monoclines are the surface expression of basement

blocks that overrode each other along high-angle reverse faults.

Kelley (l955s,-b) likewise asserted that horizontal compressive• •

forces transmitted through the basement rocks were responsible for the

monoclines of the Colorado Plateau. Kelley and Clinton (i960), in the

context of their horizontal congressional model for the origin of mono­

clines, suggested that the regional trends of the monoclines can be used

to determine the orientations of the forces which produced the folding.

They assumed that the horizontal directed forces responsible for the

monoclines must have been oriented normal to the monoclinal trends.

Given both NE- and NW-trending monoclinal sets in the Colorado Plateau,

they postulated two periods of deformation. During the first phase

forces were oriented NE-SW, thus producing the NW-SE trending monoclines;

during a second phase, compressions! forces were oriented NW-SE, pro­

ducing the NE-SW trending monoclines. They were unable to account for

the N-S trending monoclines, including the Organ Bock monocline examined

in the report. For example, they noted "In the above analysis the 8 northerly trending Defiance and a portion of the Comb Ridge monoclines were not mentioned. Although they might be a separate diastrophic phase,

it is preferable for the present to include them with the first phase

(northwesterly). However their relationship to the northeasterly-trending

monoclines is not clear.” (Kelley and Clinton, I960, p. 97)»

Woodward (1973) proposed that strong west to east and south to

north directed compressive forces resulted in a northeasterly shift of

the Colorado Plateau which "caused the monoclines” (Woodward, 1973» P* 97)

Edmonds (1961) studied the deformational characteristics of the

west-southwest-facing Nutria monocline which forms the southwestern

boundary of the Zuni uplift. He interpreted the monoclinal folding

through tangential copipression and attendant counter-clockwise rotation

of the middle limb.

Relationship of Monoclines to Basement Structures

The relationship, if any, of basement structures to monoclines

is important in any attempt to explain the origin of monoclinal folds.

However, there are few places within the Colorado Plateau where this base­

ment structure may be observed directly. The classic locality to observe

this structure is in the Grand Canyon. Babenroth and Strahler (1945)

reported that the East Kaibab monocline may be observed at the Grand

Canyon to pass with depth into a high-angle reverse fault. More recent

workers in the area (Maxson, 1961; Huntoon, 1969, 1971, 1974; and Huntoon

and Sears, 1975) have described the same basement structures. Moreover,

they have reported that recurring movements took place along these faults 9 from Precambrian to Tertiary time. The transition from monoclines to high-angle reverse faults is accommodated by a steepening in dip and a narrowing of the width of the middle limbs with increasing depth. Al­

though the ages of the faulting are somewhat ambiguous, it appears cer­

tain that the basement faults were first active in the Precambrian

(Huntoon and Sears, 1975; and Masson, 1961). Shoemaker, Abrams, and

Squires (1974) also suggested that many of the northeasterly-trending monoclines of the Colorado Plateau pass with depth into faults of prob­

able Precambrian age.

Luchitta (1974) suggests that zones of weakness in the basement,

some of which may be very old, have localized deformation from different

stress regimens, and that compressive stress in the Laramide resulted in

reverse faulting in the basement and corresponding folding in the over-

lying sedimentary rocks.

Case and Joesting (1972) made a geophysical study of the Central

Colorado Plateau and attempted to relate the geophysical data describing

the basement to the surficial structures. They defined a roughly polyg­

onal pattern of basement structures, some of which can be related to

monoclines and folds.

In the Central Rocky Mountains the level of erosion is much

deeper than on the Colorado Plateau and the deep structures of the up­

lifts and associated monoclines is more clearly revealed. Wise (1963),

Prucha et al. (1965), and Stearns (1970) all describe high-angle reverse

faults either singly or in fan array bounding the crystalline basement

blocks, while Berg (1962) described low-angle overthrusts bounding others.

Wise (1963) described keystone grabens developed in the sedimentary rocks 10 above the basement faults in the monoclinally folded section. Prucha et al. (1965) described various types of tensional structures in the

sedimentary rocks draped over the edges of the uplifted blocks.

Steams (1970) was concerned with the style of deformation in the area of the bounding faults, and determined that two styles of defor­ mation characterize this zone: the crystalline basement is seen to be deformed by brittle failure, while the sedimentary cover rocks deform by

tensional faulting, flexural flow, and flexural slip. He described de^-

collement zones found in some instances either between the sedimentary units and the crystalline basement blocks, or just above this interface.

Age of the Monoclinal Folding

It is difficult to precisely date the age of the monoclinal fold­

ing on the Colorado Plateau. Due to the Tertiary uplift and widespread

erosion, many of the uppermost units that would be necessary for placing

time constraints on folding have been removed. Kelley (1955a) cites

several authors who have described Eocene beds unconformably overlying

the beveled middle limbs of monoclines whose youngest beds are upper

Cretaceous. Gilbert (1877) documented the same type of beveled erosion

surface unconformably overlain by Eocene units on the Waterpocket

monocline.

Huntoon (1974), in discussing the Laramide and post-Laramide

structural geology of the eastern Grand Canyon area, pointed out that

several periods of movement have taken place along the faults cutting

the Precambrian rocks beneath the monoclines. This implies that dif­

ferent movements at different times may have increased or decreased the 11 structural relief on the monoclines. Barnes (1974) described the struc­ tural geology of the Grey Mountain area near Cameron, Arizona, and sug­ gested that the monoclines there did not form synchronously. He postulated that some activity may have taken place along the monoclinal trend during both Triassic and Neogene time. Shoemaker (1956) has sug­ gested that the monoclines on the western portion of the Colorado Plateau formed first, in the upper Cretaceous(?), and that they became progres­ sively younger to the east, with deformation ending in the Eocene. Thus, the available data, although limited, place the time of monoclinal fold­ ing and the rise of the major uplifts of the Colorado Plateau between upper Cretaceous and Eocene. STRUCTURAL GEOLOGY

Folds

Kelley (1955b) undertook an exhaustive study of the monoclines of the Colorado Plateau tectonic province. In this work he describes a monocline as any local, abrupt steepening in the dip of essentially horizontal strata, regionally persistent along strike. By this defini­ tion, the major fold structures in the study area can be considered to be monoclines. Generally, the form of a monocline can be described with respect to a tripartite morphology (fig. 3). The structurally highest segment, herein referred to as the upper limb, is a broad, gently dip­ ping plane that may attain a length of 50 kilometers or more, normal to the trend of the axis of the monocline. Below this and separated by the upper hinge is the middle limb. This segment is much shorter than the upper limb, but attains steeper dips. The structurally lowest segment

is the gently dipping lower limb, which in some cases may serve as the upper limb for an adjacent monocline; such is the case in the study area where the lower limb of the Organ Rock monocline forms part of the upper

limb of the Comb Ridge monocline (fig. l).

The major folds in the study area are three intersecting mono­

clines which form a characteristic "wishbone" morphology (figs. 1 and 2,

in pocket). The photogeologic expression of these folds can be seen in

the LANDSAT-1 imagery used for the base of figure 2. The physiographic

expression of the monoclines in the study area is characterized by homo-

clinal ridges (Thorribury, 1962, p. 135) whose apices point toward the

12 upper hinge

lower King

upper limb I middle limb lower limb

Figure 3. Schematic Diagram of a Monoclinal Fold Showing the Spatial and Geometric Relationships of the Limbs and Hinges• G 14 point toward the upper limb. The upper and middle limbs are cut by- entrenched streams whose canyons diminish in depth toward the lower limb. This particular physiographic expression is particularly useful in locating the positions of the hinges on small-scale air or satellite photos.

The first of the monoclines to be discussed is the Organ Rock monocline. This monocline trends N5°E through the western portion of the study area (figs. 1 and 2), faces east and displays a maximum middle- limb dip of 37°E. Close to the northern boundary of the study area the monocline swings to N30°E for 3*2 km (2.5 mi) then returns to its north­ erly trend beyond the limits of the study area (figs. 1 and 2). The lower limb of this monocline, which forms a part of the upper limb of the Comb Ridge monocline, has an average bedding attitude of N10°W (fig.

4)« In the study area the lower hinge is coincident with the Oljeto Sag of Baker (1936) and Kelley (1955a). This syncline is lost beneath the alluvium 7 km (5.6 mi) northeast of Marsh Pass. The upper hinge of the

Organ Rock monocline corresponds to an anticline produced by the juxta­ position of the gently westward-dipping strata of the upper limb and the more steeply eastward-dipping strata of the middle limb (figs. 5a,b,

6). This type of morphology is characteristic of many of the monoclines of the Colorado Plateau tectonic province (Davis and Kiven, 1975; Kelley,

1955b). To the south, in the vicinity of Marsh Pass, the Organ Rock monocline swings to the southwest and joins the N55°E-trending Cow

Springs monocline (fig. l). This junction will be referred to herein as

the Kayenta salient. Although there is no structural discontinuity 15

Kilometers 110= 45 "

Figure 4* Lower-Hemisphere, Equal-Area, Pole-Density Diagram of Poles to Strata that Comprise the Lower Limb of the Organ Rock Monocline and the Upper limb of the Comb Ridge Monocline.

See figure 2 for geographic references. 16

Figure 5» Photographs of the Organ Rock Monocline.

(a). Photograph of the middle and lower limb strata, to become lower limb strata, looking S10 w. 17

Figure 5» continued.

(b). Upper hinge of the Organ Rock monocline. Strata in the upper limb on the left hand side of the photograph dip gently away from the middle limb strata. Photograph taken looking about N5 E. 18

120 Readings

Figure 6. Lower-Hemisphere, Equal-Area, Pole-Density Diagram of Poles * to Strata in the.Middle and Upper limbs of the Organ Rock Monocline.

See figure 2 for geographic references. 1.9 between the two monoclines, they have been customarily identified as two separate entities (Kelley, 1955a; Kelley and Clinton, I960).

The Organ Rock monocline has been breached by erosion to expose rocks of the Glenn Canyon Group, Chinle Formation, Moenkopi Sandstone and Cutler Formation in the upper and middle limbs and the Glenn Canyon

Group and Chinle Formation in the lower limb (fig. 1, table l). The degree of exposure has aided in the determination of the structural re­ lief produced by the monoclinal folding. At the Kayenta salient the structural relief is 985 m (3150 ft). This decreases to 460 m (1500 ft) at the northern border of the area. Beyond this point the structural relief gradually diminishes until the monocline dies out 65 km (52 mi) north of the Kayenta salient in the vicinity of the San Juan River.

The next monocline to be described is the Cow Springs monocline, the northeastemmost portion of which is located in the extreme south­ west comer of the study area. This monocline is represented by a 3.5 km

(2.8 mi), N55°E-trending, southeast-facing segment which attains a maxi­ mum dip of 30° (fig. ?)• The upper hinge corresponds to that area where

the upper-limb strata begin to strike northwest. A notable character­

istic of the upper hinge is its expression as several gently southwest-

plunging anticlines and synclines. These folds begin 5 kilometers north

of the Kayenta salient and continue southward in a roughly en echelon

pattern to become the upper hinge of the Cow Springs monocline (fig. l)

No syncline marks the lower hinge of the Cow Springs monocline; there­

fore, the author has chosen to arbitrarily define the lower hinge as

that zone where the strata begin to dip consistently less than 5°« The

Cow Springs monocline passes 50 kilometers (40 miles) beyond the study 20

137 Readings

Kilometers

Figure ?• Lower-Hemisphere, Equal-Area, Pole-Density Diagram of Poles to Strata in the Middle limb of the Cow Springs Monocline.

See figure 2 for geographic references. 21 area to the vicinity of Red Lake (fig. 2), where it passes beneath alluvium.

Rocks exposed in the upper limb are predominantly units of the

Glenn Canyon Group and the upper members of the Chinle Formation. The

Glenn Canyon Group is overlain unconformably by a heavily forested ve­ neer of dune sand. In the middle limb, erosion has exposed strata that range from the Triassic Chinle Formation to the Late Cretaceous Mesa

Verde Formation (table l). However, recent debris flows conceal much of the outcrop above the Glenn Canyon Group and below the upper portions of the Mancos Shale (fig. 1). Strata comprising the lower limb include formations from the Jurassic Morrison Formation to the upper Cretaceous

Mesa Verde group. No strata younger than the Mesa Verde group are in­ volved in the monoclinal folding in the study area.

The structural relief on the Cow Springs monocline reaches a maximum of 985 m (3150 ft) at the Kayenta salient. From this marimum it steadily decreases to the southwest until west of the trading post at

Cow Springs the structural relief is 5&0 m (1800 ft). Farther to the

southwest where the monocline passes beneath recent alluvial deposits,

the structural relief can only be estimated at less than 350 m (1200 ft).

Beaumont and Dixon (1965) published a series of stratigraphic

sections from the Kayenta and Chilchinbito quadrangles. The thickness of

the Mancos Shale and the Morrison Formation (table 1) vary by as much as

II5S and 2yfo between the middle and lower limbs of the Cow Springs mono­

cline, with the middle limb showing the thinning. The area in which the

sections were measured is within 20 km (16 mi) of the point where

Harshbarger et al. (1957) show the Cow Springs sandstone, a fine-grained 22 sandstone with silt and clay lenses, in the same stratigraphic interval occupied by the Morrison Formation in the study area. Both sets of workers consider these two units to be facies equivalents. It is pos­ sible that some of the 23% thinning reflects the facies change.

The third monocline in the study area is the Comb Ridge mono­ cline which trends N60°E and faces to the southeast (fig. 8). The maxi­ mum dip of the middle-limb strata is 27°• One of the most notable characteristics of this monocline is the striking angular discordance between the strike of the upper- and middle—limb strata* This can be seen in figure 1, and through comparison of the lower-hemisphere, equal- area, pole-density diagrams for bedding (figs. 4 and 8). This abrupt change in the bedding attitudes is similar to that sometimes seen asso­

ciated with faulting; however, no faults exist here capable of producing

the observed change in bedding attitudes. As in the case of the Cow

Springs monocline, there is no syncline closely associated with the Comb

Ridge monocline to serve as the lower hinge. Therefore, the lower hinge

is placed through the array of points marking where the strata begin to

dip consistently less than 5°*

Rock units exposed in the Comb Ridge monocline include the Chinle

Formation and Glenn Canyon Group on the upper and middle limbs. The

lower limb is almost totally covered with alluvium, although some small

outcrops of Glenn Canyon Group and Morrison Formation are shown in fig­

ure 1.

The maximum structural relief on the Comb Ridge monocline is

along the north-trending portion of the monocline, 115 kilometers (92

miles) to the northeast of the Kayenta salient, near Bluff, Utah. There 23

121 Readings

110*45'

Figure 8. Lower-Hemisphere, Equal-Area, Pole-Density Diagram of Poles to Strata in the Middle Limb of the Comb Ridge Monocline.

See figure 2 for geographic references. 24 the structural relief reaches 1095 m (3500 ft). This displacement de­ creases until it reaches 310 m (1000 ft) as the monocline passes be­ neath the alluvium of Laguna Creek, 8 km (6.4 mi) east of the Kayenta salient.

Isolated outcrops indicate that this monocline is united with the Organ Rock and Cow Springs monoclines. The lower hinge and lower limb of the Cow Springs and Comb Ridge monoclines are coincident, and the same strata are involved in the upper limb of the Comb Ridge mono­ cline as in the lower limb of the Organ Rock monocline (fig. l). Fur­ thermore, the width of the Cow Springs monocline is approximately equal to the combined structural relief on the Comb Ridge and Organ Rock monoclines east and north of their junction at the salient.

The only other fold in the area is a small southeast-plunging symmetrical anticline 2 km (1.6 mi) southeast of the Kayenta salient

(fig. l). This fold is very subtle; limb dips are between 5° and 10°.

The wavelength is approximately 2 km (1.6 mi) and the amplitude is ap­ proximately 100 m (320 ft). While this anticline is evident in the

Jurassic and Cretaceous rocks southeast of the Kayenta salient, it is not apparent in the massive sandstones of the Triassic-Jurassic Glenn

Canyon Group. It dies out approximately 5 km (4 mi) to the southeast of the lower hinge of the Cow Springs and Comb Ridge monoclines.

Joints

For the purpose of this discussion the description of joint ele­ ments is divided into two parts. The first is a description of the physical characteristics of the joint elements; the second is a geometric 25 description of joint elements by domains. The domains are characterized by homogeneous bedding attitudes with boundaries delineated by the axial traces of the monoclines. Because the frequency of jointing in an area of simple stratigraphy and structure may be a function of the thickness and lithologic characteristic of the rock units (Harris, Taylor, and

Vfalper, i960), the joint analysis was restricted to the massive sand­ stone units of the Glenn Canyon Group which crop out throughout the study area (fig. 1, table 1).

The following generalizations regarding joint elements in the study area may be made:

1) joints show no systematic pattern of spacing;

2) long continuous joints (greater than 5 m in outcrop trace)

are often paralleled by shorter discontinuous joints;

3) members of joint sets may terminate at lithologic boundaries

or they may cross those boundaries;

4) there is no systematic pattern by which members of any one

joint set can be shown to consistently terminate against mem­

bers of any other joint set.

Joints were analyzed in two domains containing rocks that were not involved in the monoclinal folding. One area, at Navajo National

Monument, 18 km (11 mi) west of the study area, was analyzed to deter­

mine if the geometries of the joints in rocks unaffected by monoclinal

folding are consistent on a regional scale. Bocks in this area are

flat lying and are 19 km (12 mi) from the nearest monocline. The

stereonet in figure 9 displays the orientations of all joints measured 26

102 Readings

Kilometers

Figure 9* Lower—Hemisphere, Equal-Area, Pole—Density Diagram of Poles to Joints Collected in the Betatakin Area.

Joint measurements were made in the Navajo sandstone, Glenn Canyon Group. See figure 1 for geographic references. 27 in this domain. The dominant joint sets in this area which strike

N15°W, N15°E and N50°E are all nearly vertical. Although these modes seem dominant, it should be noted that almost all strike directions are represented in this domain.

The second control area in which joints were analyzed is in the north-central portion of the study area in Narrow Canyon 2 km (1.6 mi) east of the lower hinge of the Organ Rock monocline (fig. l). This canyon cuts through strata that are common to the lower limb of the

Organ Rock monocline and the upper limb of the Comb Ridge monocline.

Steep cliffs in Narrow Canyon offer excellent three-dimensional expo­ sures for the collection of joint data. Four dominant joint sets were identified in this area. These sets strike N10-15°W, N40OW, east-west, and N75°Ej dips are 80-90° (fig. 10).

The primary sites chosen for the geometric analysis of the at­ titudes of joints with respect to the attitudes of the strata containing them are in the middle limbs of the Comb Ridge, Organ Rock, and Cow

Springs monoclines, respectively. A lower-hemisphere, equal-area, pole- density diagram of joints in the middle limb of the Comb Ridge monocline is shown in figure 11. This plot indicates three dominant joint sets at

N20°E vertical, N85°N 6$°NE and N40°E 75°NW. By far the majority of the joints dip to the northeast and northwest toward the upper limb of the monocline. Those joints which strike most nearly parallel to the trend of the middle limb of the monocline appear to attain the shallowest dips (fig. 11). 28

57 Readings

Kilometers 110*45'

Figure 10. Lower-Hemisphere, Equal-Area, Pole-Density Diagram of Poles to Joints Measured in Narrow Canyon in the Formations of the Glenn Canyon Group.

See figure 1 for geographic references. 29

64 Readings

Kilometers 110*45

Figure 11. Lower-Hemisphere, Equal-Area, Pole-Density Diagram of Poles to Joints from the Middle limb of the Comb Ridge Monocline.

Joints were measured in the formation, of the Glenn Canyon Group. See figure 1 for geographic references. 30

.Three dominant joint sets were recognized in the middle limb of the Organ Rock monocline (fig. 12). The attitudes of these joint sets are N70°W vertical, N55°W 80°SW and N70°E 80°NW. Joints of the N70°W set are the most continuous and may be traced in outcrop for distances approaching 50 meters (l60 feet).

Joints in the middle limb of the Cow Springs monocline include four dominant sets at N65-70°E 70° NW, N45-50°E 60°NW, N45-50°W vertical and N15-20°E 60°NW (fig. 13). Note that the pole-density diagram indi­ cates that most of the joints dip NW toward the upper limb, and as seen in the pole-density diagrams for joints in the two preceding monoclines, the joints that strike most nearly parallel to the trend of the middle

limb are the steepest (fig. 13).

Faults

Faults appear to be restricted to the middle limbs of the mono- i i clines in the study area. No fault with a displacement of greater than

5 meters (l6 feet) was discovered in the entire study area. The majority

of faults display only a few centimeters of displacement (figs. U+a and

14b), while an estimated 5$ of approximately 175 faults examined in the 1 area have a displacement greater than 1 meter. The longest fault trace

measured is approximately 100 meters (320 feet). All faults are too small

to be shown on figure 1, the geologic map.

The faults may be divided into two categories on the basis of

their physical and geometric characteristics: l) those that cut across

and displace primary lithologic boundaries, i.e., bedding and separation

planes; and 2) those that parallel those boundaries. The discordant 31

56 Readings

Kilometers

Figure 12. Lower-Hemisphere, Equal-Area, Pole-Density Diagram of Poles to Joints from the Middle limb of the Organ Rock Monocline.

The data were collected from formations of the Glenn Canyon Group. See figure 1 for geographic references. 32

Kilometers

Figure 13• Lower-Hemisphere, Equal-Area, Pole-Density Diagram of Poles to Joints Measured in the Formations of the Glenn Canyon Group Exposed in the Middle Limb of the Cow.Springs Monocline.

See figure 1 for geographic references. 33

(a). Displacements in aeolian crogs-bedded wingate sandstone. Photograph taken looking about N50 E at a steeply-dipping slope in the middle limb of the Comb Ridge monocline.

(b). Displacements in the middle limb of the Organ Rock monocline. Photograph taken looking N10 E.

Figure 14. Discordant Faults Showing Minor Displacements 34 faults are planar, have steep dips (rarely less than 60°) and dip pre­ dominantly toward the upper limb (figs. 15, 16 and 17). These faults are commonly marked by a thin layer of gouge or crystalline calcite and may show slickensiding and chatter marks (figs. 18 and 19). Although the gouge-bearing faults rarely reach a thickness of greater than a few centimeters, the calcite-filled faults locally attain 10 cm (4 in) in thickness. Occasionally a planar calcite-bearing fault is observed to be cut, but not offset, by secondary fractures that are less than 0.5 m in length and oriented at 25-30° to the fault (figs. 20 and 21).

The kinematics of the faults are difficult to assess due to the minor displacements of lithologic boundaries and because slickensides indicate dip-slip, strike-slip, or oblique-slip motion. Because the kinematics of the faults are obscure, no slip or separation classifica­ tions may be applied with a high degree of confidence. Of the few faults where relative displacements are unequivocally documented, the majority may be classified as high-angle reverse faults often with a component of strike-slip motion. In general, where motion can be determined, the blocks closest to the lower hinge are down-dropped.

Although there does not appear to be a systematic spacing pat­ tern for the observed faults, a conservative estimate would be an aver­ age of three to four faults per 100 square meters within the middle limbs of the monoclines.

The faults have a characteristic geometric pattern with respect to each monocline similar to that seen for the joints associated with the same monoclines (figs. 11, 12, 13, 15, 16 and 17). Therefore, to 35

Kilometers

Figure 15• Lower-Hemisphere, Equal-Area, Pole-Density Diagram of Poles to Discordant Faults Measured in the Middle T.imh of the Comb Ridge Monocline.

See figure 1 for geographic references. 36

Figure 16. Lower-Hemisphere, Equal-Area, Pole-Density Diagram of Poles to Discordant Faults in the Middle Iamb of the Cow Springs Monocline.

See figure 1 for geographic references. 37

37 Readings

Kilometers IIO°45'

Figure 17 • Lower-Hemisphere Plot of Poles to Discordant Faults in the Organ Rock Monocline.

See figure 1 for geographic references. 38

Figure Id. Oblique-Slip Slickensides and Polish on Discordant Fault Surface in Middle Limb of Organ Rock Monocline.

Slickensides plunge gently S50°E. Figure 19. Chatter Marks and Slickensides on Large Block of Kayenta Sandstone from the Middle Limb of the Comb Ridge Monocline.

Note striations on both faces of the block. 40

N

Figure 20. Sketch Showing Full Relationships of Cross-Fractures on Calcite-Bearing Discordant Fault in Kayenta Formation.

Figure 21. Cross-Fractures on Calcite-Bearing Discordant Fault in Kayenta Formation in Middle Limb of Comb Ridge Monocline.

Main fault striking N70°W across right-hand corner of photograph is dashed in ink. Note pencil for scale. 41 simplify this discussion, the geometries of the discordant faults will be described by domains. The boundaries for these domains are the same used for the discussion of the joint elements.

Figure 15, which shows the attitudes of the faults in the mid­ dle limb of the Comb Ridge monocline, is dominated by three modes.

These modes represent faults that strike N50°W 60°NE, N80°W 65°NE and

N60°E 70°NW.

In the Cow Springs monocline two dominant fault sets may be identified (figure l6). One represents vertical faults that strike

N40°W and are nearly normal to the trend of the middle limb. The sec­ ond set of faults strikes from N85°W 65°NE to N55°E 60°NW, with a den­ sity maximum centered close to N85°W 65°NE.

Because of insufficient measurements from the middle limb of the Organ Rock monocline, it was not possible to prepare a statistically valid pole-density diagram. Instead, poles to fault surfaces are plot­ ted in figure 17• In general, the faults cut the middle limb at nearly all strike directions. However, it is clear from figure 17 that the majority of the faults dip to the northwest and southwest..

One geometric characteristic is common to the discordant faults in the above-mentioned domains. Examination of figures 15, 16 and 17

shows that those faults which strike most nearly parallel to the mid­

dle limb trends of the monoclines in which they were observed have the

shallowest dips. In addition, those faults whose attitudes strike

nearly normal to the trend of the monoclines containing them display

the steepest dips. In all cases the great majority of faults can be 42 shown to dip toward the upper limbs of the monoclines with which they are associated. Furthermore, comparison of the stereograms for faults and joints in the respective middle limb zones will show that this geo­ metric characteristic is common to both types of structure.

Although discordant faults cut all rock units, the second cate­ gory of faults, the concordant faults, occurs almost exclusively in the aeolian cross-bedded units. These faults are characterized as being curviplanar and confined to parting surfaces. The fault surfaces are identified by gouge and slickensides (figs. 22 and 23); none were observed to be calcite-bearing. Furthermore, the faults have a partic­ ular geometric relationship to the strata of the middle limbs of the monoclines in which they occur. Poles to faults in the aeolian cross- bedded units in the middle limb of the Comb Ridge monocline are repre­ sented by a single dense mode at N35°E 40oSE (fig. 24). This mode is nearly identical to the mode for the poles to dune surfaces and bed­ ding planes in these units (fig. 25). Thus, the faults in the aeolian

cross-bedded units parallel parting surfaces. 43

Figure 22. Gouge-Bearing Concordant Fault in Aeolian Cross-Bedded Navajo Sandstone in the Middle limb of the Cow Springs Monocline.

Note how bedding above the gouge zone becomes tangential to the gouge zone. Photo taken looking N6CTE. a

Figure 23. Slickensides on Concordant Fault Surface in the Navajo Sandstone in the Middle Limb of the Comb Ridge Monocline.

This photograph was taken looking up at the base of a dune surface. Slickensides plunge 35°S 43°Et nearly normal to the trend of the middle limb. 45

63 Readings

Kilometers 110*45"

Figure 24* Lower—Hemisphere, Equal—Area, Pole—Density Diagram of Poles to Aeolian Cross—Bed Surfaces in the Navajo Sandstone from the Middle limb of the Comb Ridge Monocline.

See "figure 1 for geographic references. 46

53 Readings

iio m s '

Figure 25• Tower-Hemisphere, Equal-Area, Foie—Density Diagram of Poles to Concordant Faults from the Navajo Sandstone from the Middle Limb of the Comb .Ridge Monocline. ,

See figure 1 for geographic references. DISCUSSION

Introduction

The majority of the models concerning the origin of the monoclinally-bounded uplifts of the Colorado Plateau tectonic province suggest that a large component of crustal shortening accompanied their formation. For example, Shoemaker (1956) compares the deep structure of the uplifts to the overriding of blocks in an iceflow. The shortening is attributed to a horizontally directed compressive stress, transmitted through the basement and sedimentary rocks, and oriented normal to the middle limbs. Viewed in this way, the Cow Springs and northeast- southwest trending segment of the Comb Ridge monoclines in the study area would be interpreted as a product of northwest-southeast compression

(Kelley and Clinton, I960). But this orientation is not compatible with the suggested northeast-southwest directed orientation of the maximum regional compressive stress during the Baramide (Coney, 1972).

It is possible, however, to interpret the kinematics of forma­ tion of the monoclines in the study area in a way that is not dependent on a large component of horizontal compression and attendant large-scale

crustal shortening throughout the entire section. Instead, a model

similar to those proposed by Thom (1955) or Prucha et al. (1965) in­ volving a strong component of vertical motion along high-angle reverse

faults is suggested.

47 48

Basement Structures Associated with Uplifts

In order to define the kinematics of movement for the southwest comer of the Monument uplift, it is first necessary to discuss the deep structure of the monoclihally-bounded uplifts. For the purpose of this paper, "deep structure" refers to those structures which are found in the Precambrian crystalline rocks beneath the uplifts.

At those locations in the Colorado Plateau where the deep structure of an uplift is exposed, high-angle reverse faults are situ­ ated beneath and parallel to the bounding monocline(s) (Huntoon, 1969;

Huntoon and Sears, 1975» Kelley, 1955b; and Edmonds, 1961). Due to the structural similarity among monoclines in the Colorado Plateau, the as­ sumption has been made (Kelley, 1955a,-b; and Shoemaker, 1956) that most of these monoclines are situated above high-angle reverse faults in the

Precambrian crystalline basement. In the Grand Canyon region these faults have been documented as reactivated Precambrian faults (Huntoon and Sears, 1975)• While the sense of movement during different periods of deformation has varied, it appears that a reverse movement in the

Laramide(?) resulted in monoclinal folding of the overlying sediments

(Maxson, 1961; Huntoon and Sears, 1975)• The dip of the inferred base­ ment faults is commonly suggested to be toward the structurally highest limb of the monocline, while the strike, by necessity, is parallel to the trend of the middle limb.

Stearns (1970), in discussing the high-angle reverse faults in the deep structure of monoclines in the Central Rocky Mountains, noted that if a strictly planar fault were assumed to have produced the 49 observed tilting of the uplifted basement blocks, stress would be con­ centrated along one comer of the block. This would result in intense crushing deep within basement and the creation of a void near the basement-cover contact (fig. 26a). However, if a curviplanar fault were assumed, then the observed tilting could be induced (fig. 26b).

That high-angle basement faults form the deep structure of mono­ clines has been inferred for many parts of the Colorado Plateau tectonic province. Kelley (1955a) and Kelley and Clinton (i960) include the Cow

Springs and northeast-trending segment of the Comb Ridge monoclines as portions of the ’*Coconino lineament". The southern portion of this lineament is the Mesa Butte fault system and the Additional Hill mono­ cline. These workers infer that the "Coconino Lineament" is the sur­ face expression of a high-angle basement weakness that localized deformation in the Laramide(?). The lineament is parallel to the northeasterly-trending basement fractures of Shoemaker et al. (1974).

Moreover, Shoemaker et al. (1974)» using the Residual Aeromagnetic Map of Arizona (Sauck and Sumner, 1971) have identified a clear magnetic anomaly that represents a discontinuity in the basement rocks that par­ allels the entire length of the "Coconino Lineament" in Arizona. Addi­ tional geophysical evidence for basement fractures in the central

Colorado Plateau is presented by Case and Joesting (1972). Specifi­ cally, their work interprets the residual aeromagnetic and gravity data associated with the entire Comb Ridge monocline to indicate faulting in the crystalline basement rocks. 50

Figure 26a. Schematic Cross-Section of Basement Structure Showing how a Planar Fault would Produce Stress or Volume Abnormalities (after Steams, 1970).

Figure 26b. Schematic Cross-Section of Basement Structure Showing how a Curviplanar Fault would Produce a Tilted Basement Block without Volume or Stress Abnormalities (after Steams, 1970). 51

Davis (1975) defined four sets of high-angle basement struc­ tures on the basis of alignments and trends of the middle limbs of the monoclines of the Colorado Plateau. These zones appear to divide the crystalline Precambrian basement into a series of polygonal blocks.

They strike northwest, north-northwest, north-northeast and northeast.

Inferred Basement Structures in the Study Area

The independent lines of geologic and geophysical data pre­ sented above would seem to indicate that each of the monoclines in the study area (the Organ Rock, Cow Springs and Comb Ridge monoclines) is associated with high-angle basement faults. Figure 2? is a reproduc­ tion of a portion of the Residual Aeromagnetic Map of Arizona (Sauck and

Sumner, 1971)• The "Coconino lineament" can be identified trending northeastward across this figure. In the study area a secondary mag­ netic gradient is shown which parallels the north-south trend of the

Organ Rock monocline. Cradled in the "Y" formed by the intersection of the three monoclines is a strong magnetic high. This anomaly can be interpreted with a high degree of confidence as a pluton or series of plutons at depth (J. S. Sumner, The University of Arizona, personal com­ munication, 1975)*

A portion of the Residual Gravity Map for Arizona (West and

Sumner, 1973) that includes the study area is reproduced in figure 28.

A gravity gradient can be identified paralleling part of the Organ Rock monocline, while another gradient is shown parallel to the middle limbs of the Comb Ridge and Cow Springs monoclines. Combined with the data

shown in figure 27 and the abrupt flexures in the surficial rocks, these 52

Figure 27 • Reproduction of a portion of the Residual Aeromagnetic Map of Arizona (Sauck and Sumner, 1971)•

The traces of the hinge lines of the three monoclines in the study area are plotted to show the parallelism of the surface structures and base­ ment discontinuities. Dashed line indicates position of Coconino lineament. See figure 1 for geographic references. 53

no 45

• .. \

Figure 28. Reproduction of the Residual Gravity Map of Arizona (West and Sumner, 1973)•

The traces of the hinge lines of the three monoclines in the study area are plotted to show the parallelism of surface structure and basement anomalies. Dashed lines indicate position of gravity gradients. See figure 1 for geographic references. 54 data strongly suggest that there are faults in the basement beneath and parallel to the monoclines.

The presence of dikes and plugs of augite minette in the study area (fig. 1; table 1) is consistent with the interpretation that high- angle, deep seated faults underlie each of the monoclines. Eastwood

(1974) suggests that the magmas which formed these structures originated in the upper mantle or lower crust. He felt that the localization of volcanics in the southern Colorado Plateau was due to zones of weakness, in the deep crust and faults in the uppermost crust. If correct, this would imply that the volcanics seen in the study area may have been localized by faulting in the upper crust and that the faults extended into the lower crust or upper mantle as zones of weakness.

Given the data and inferences presented above, the basement structure of the study area can be visualized as being dominated by three large fault-bounded blocks. The northwestern block, herein re­ ferred to as the Tsegi block, is bounded on the east by the Organ Rock fault and on the southeast by the Cow Springs fault. The surface ex­ pressions of these faults are the Organ Rock and Cow Springs monoclines, respectively. To the east of the Tsegi block and separated from it by the Organ Rock fault is the Monument block, downthrown with respect to the Tsegi block. Maximum structural relief, estimated across the Organ

Rock fault, is 810 meters (2600 feet). The southeastern boundary of the

Monument block is the Comb Ridge monocline. To the southeast and down- thrown with respect to the Tsegi and Monument blocks is a third basement block, herein referred to as the Black Mesa block. This block utilizes 55 both the Cow Springs and Comb Ridge fracture zones as its northern boundary. In the vicinity of the Kayenta salient the structural relief between the Monument and Black Mesa blocks is approximately 310 meters

(1000 feet) and between the Tsegi and Black Mesa block, 985 meters

(3150 feet).

Kinematics of Basement Block Movement

In order to provide a clearer picture of the Laramide(?) tec­ tonic history of these blocks, a structural relief map (fig. 29a) was generated for each of the monoclines in the study area. Because the monoclines are inferred to mark the edges of the basement blocks, the structural relief of the monoclinally folded sedimentary strata reflects the relative structural relief between the basement blocks. Structural relief between the upper and lower limbs was determined for a number of locations along the length of the monoclines. Data were compiled from structure contour maps by Beaumont and Dixon (1965)» Witkind and Thadden

(1963), Vogel (1963), Sears (1956) and Baker (1936), and were plotted on an overlay derived from figure 2 in Kelley (1955a)• For areas where no structure contour maps are available, structural relief was deter­ mined by generating cross sections from Cooley et al. (1969)•

An interesting feature of figure 29a.is the arcuate shape of the monoclinally-bounded uplifts. This shape may represent the surface ex­ pression of two or more high-angle basement faults. Figure 29b shows the strikes of the inferred basement faults and the relative motions on each fault. The structural relief along the faults (fig. 29a) increases more or less constantly from the terminal ends of both the Cow 56

I Monticello I

I l

AXES OF ROTATION

l

UTAH______ARIZONA

AXES OF 370 ROTATION Kayento

a

b

Figure 29• Structural Relief Map.

(a) . Map generated for the monoclines that form the Kayenta salient, lines connecting points of equal structural relief on the monoclines have been shown to represent axes of natation for the basement blocks. See text for data source.

(b) . Inferred orientations of basement structure beneath the Organ Rock, Cow Springs and Comb Rid^o monoclines showing relative motions. 57

Springs-Organ Rock monoclinal system^ and the Comb Ridge monoclonal system. By determining the change in structural relief per unit of dis­ tance parallel to the middle limb a structural gradient can be defined for each segment. For the Comb Ridge monoclinal system this gradient is approximately 8 meters/kilometer (42 feet/mile) for both the north­ easterly and north-northwest trending segments, and 6 meters/kilometer

(32 feet/mile) for the central north-northeasterly trending segment.

The structural gradients computed, for the Cow Springs-Organ Rock mono­ clinal system are 8 meters/kilometer (42 feet/mile) for the northeasterly trending segment, 5 meters/kilometer,(30 feet/mile) for the north- northeasterly trending segment, and 16 meters/kilometer (98 feet/mile) for the northerly-trending segment. This general increase in struc­ tural relief, which reaches a maximum near the center of the systems, •

allows axes of rotation to be defined to describe the movement of the

Monument block with respect to the Tsegi block. These lines represent

hypothetical axes about which the basement blocks have been rotated to

produce the observed structural relief along the monoclinal segments.

Using these lines and ••points'* of maximum structural relief, axes and

directions of rotation may be defined. Through this procedure, axes of

rotation of N35°E for the Tsegi block and N15°E for the Monument block

were determined. For both blocks the rotation is east side up or

counterclockwise when viewed from south to north. This rotation is

• 1. The term monoclinal system is used here to describe any seg­ ments of a physically continuous monocline that have been given separate names based only on the difference in the trend of individual segments; i.e., the Additional Hill Coconino Point-Grand View monoclinal system (fig. 2). 58 what would be expected if the basement faults were curviplanar and dip back towards the upper limb, i.e., high-angle reverse faults. This hypothesis assumes that the Black Mesa block, which did not begin to receive excess sediments until Cenozoic time, was a passive feature formed by the uplift of the surrounding areas (Elston, i960), similar to the origin of the San Juan Basin (Kelley, 1957)•

If the dip of each of these faults is very steep, a small amount of lateral shortening would result in a proportionally large amount of vertical uplift. This demonstrated graphically in figure 30, which plots change in vertical displacement versus change in the dip of a hypothetical basement fault for a constant $00 m (1600 ft) of hori­ zontal displacement. Note the exponential growth in the amount of vertical displacement with increase in fault dip.

Deformation at the southwest comer of the Monument uplift can be defined on the basis of the data and interpretations presented above.

It is suggested that the basement rocks may have been separated into component units or blocks by fractures or zones of weakness formed dur­ ing previous erogenic events (Shoemaker et al., 1974; Davis, 1975)*

These fractures or zones of weakness appear to have localized stress during the Laramide(?). Moreover, the presence of high-angle reverse faults, where exposed beneath monoclines, implies that this stress was compressive. In the study area stress in the basement was accommodated predominantly by a large component of vertical movement and rotation of the basement blocks. This- could have been accomplished in two ways.

In the first alternative, compressive stress transmitted through the basement produced high-angle reverse faulting. The combined iue3. GraphofChange thein Angle Versus Vertical Displacement Figure30. of a ofFault. Note how how Note exponentially the displacementon verticalaQfault increases as the angle of the fault approaches 90 . Data were generated assuming generated were Data astheangle.of thefault 90 approaches a constant METERS 0 0 0 3 2000 0 0 5 2 0 0 I5 0 0 0 1 0 0 5 500 m of m horizontal displacement. GREES EG D 59

60

Tsegi-Monuraent block was uplifted and rotated about a northeast to north-northeast axis, utilizing the Cow Springs-Comb Ridge fault (fig.

31a). As deformation continued, the basement rocks failed along the

Organ Rock fault, and independent uplift of the Tsegi block was ini­ tiated (fig. 31b).

The alternative model switches the relative times for the ini­ tiation of uplift of the basement blocks. Uplift of the Tsegi block along the Cow Springs and Organ Rock faults would begin first, followed by the uplift of the Monument block. This alternative would be con­ sistent with Barnes (1974) and Shoemaker (1956) who suggest that mono- clinal folding began in the western portions of the Colorado Plateau and proceeded eastward, ending in Eocene time with the formation of the uplifts along the eastern boundary of the plateau. In either case, it would seem probable, given a northeast-southwest directed compression during the Laramide (Coney, 1972), that a component of strike-slip movement along the ”Coconino Lineament*' occurred during deformation.

However, there is no evidence for this in the Phanerozoic cover rocks.

Although it does not appear possible to choose between these two alternatives through examination of the surficial geology, the

former alternative appears to best fit the observed pattern of struc­

tural relief in the study area. During the first stage of deformation

(fig. 32) the relative movements of the Tsegi (A) and Monument (B)

blocks can be represented by one component of uplift, while the Black

Mesa block (C) remained stable:

A = 1; B = 1; C = 0. N

Black M e s a

Figure 31 • Block Diagram Showing Geometric Relationships of Basement Blocks.

(a). This diagram indicates how the cover rocks were draped to conform to the basement structure during the earliest stages of deformation.

H N

P / f j J h

Bloeh Mesa

Figure 31» continued.

(b). Block diagram showing the geometric relationships of the three basement blocks and the draping of the cover rocks are shown during the final stages of deformation.

ro 63

Monument (B); B= 0

Tsegi (A ), A= 1

Block Mesa (C); C=0

Figure 32. Models Showing Relative Periods of Vertical Movement for the Three Basement Blocks. Tsegi Block = (A); Monument Block = (B); Black Mesa Block = (C).

See text for detailed explanation 64

The next stage of deformation can be represented by one component of uplift for the Tsegi block and none for the Monument and Black Mesa blocks:

A = Ij B = 0} C = 0.

Summation of the components of vertical uplift between the Tsegi and

Black Mesa blocks equals the total combined relative movements between

the Monument and Tsegi blocks and the Monument and Black Mesa blocks:

Ato 0 = 2 + 0 (Bt0 A) + (Bto C) = 1 + 1 = 2.

This model agrees with the observation that the structural relief be­

tween the Tsegi and Black Mesa blocks [985 meters (3150 feet)] is ap­

proximately equal to the combined structural relief between the Monument

and Black Mesa blocks [310 meters (1000 feet)] plus the structural

relief between the Monument and Tsegi blocks [800 meters (2600 feet)].

Deformation in the Sedimentary Rocks

The vertical movements of the baseirient blocks, along high-angle

reverse faults, resulted in the draping of the Phanerozoic sedimentary

rocks over the edges of these blocks. It would be expected that this

draping would produce tensional stress in the sediments. However, the

sediments show no" large-scale tensional features. This apparent absence

of tensional features can be explained by flexural flow in the incompe­

tent units and by distribution of shear along preexisting fractures in

the competent units.

Thinning of stratigraphic units has been reported in the middle

limbs of several monoclines in the Colorado Plateau. Phoenix (1966) 65 reports thinning of the Chinle Formation in the middle limb of the Echo

Cliffs monocline, while Bradish (1952) has demonstrated thinning of units across the top of the Monument uplift. This observation is sub­ stantiated by data from well logs (Art Trevena, personal communication,

The University 'of Arizona, 1975)• The east-facing Hunters point mono­ cline (fig. 33), which bounds a portion of the Defiance Plateau (fig. 2),

'shows spectacular thinning in the beds of the Supai Formation in the middle limb. The mode of this thinning is displayed by the Mancos Shale in the middle limb of the East Defiance monocline (fig. 2). There the shale, sandwiched between massive sandstone units, exhibits a pervasive cleavage parallel to bedding. Outcrops of the Mancos Shale in the study area also exhibit a pervasive! bedding plane cleavage. It seems possible that some thinning took place in the middle limbs of the monoclines in the study area in response to basement block uplift. This assumption is supported by the stratigraphic sections of the Kayenta and Chilchiribito quadrangles (Beaumont and Dixon, 1965) which indicate apparent thinning in the middle limb of the Cow Springs monocline (see Structural Geology section).

It appears as if accommodation of extensional stress in the component units was effected by the utilization of pre-existing and syn- ‘ 1 tectonic fractures. Comparison of pole-density diagrams for joints

(figs. 10, 11 and 12) and for faults (figs. 15, 16 and 18) show similar patterns, implying similar tectonic origins. If the joints in the study area were formed prior to monoclinal folding, as suggested by Hodgson

(1961), then the faults may be explained as having been formed by the 66

Figure 33. Photograph of the Hunter’s Point Monocline near Window Rock, Arizona.

Note the apparent thinning in the beds of the Supai Formation as they pass into the middle limb of the monocline. 67

simple utilization of joints as shear surfaces. This would allow for the relaxation of tensional stress without the necessity of initiating large-scale faulting.

Two geometric tests were designed to determine if joints and

faults were rotated into their present attitudes during monoclinal fold­

ing. The first test was designed to analyze the effect of monoclinal

rotation on vertical and near vertical joint sets, hypothetical joint

sets were plotted by selecting one joint set for every ten degrees of

Compass direction from N90°E to N80OW, i.e., joint sets were selected"

striking N90°E, N80°E, N70°E, . . . N80°W. Each set consisted of three

joints of uniform strike dipping 85°W, vertical, and 85°E. An arbi­

trary hypothetical 20°E of rotation about a monoclinal axis trending

north-south was chosen. The pole-density diagram for these joints

after rotation is shown in figure 34* The effect of this rotation is

shown graphically in figure 35* This figure shows the curves generated

by plotting the number of degrees of dip after rotation as a function

of the pre—rotation angle between the strike of the joint sets and the

axis of rotation, i.e., the trend of the middle limb. The three curves

shown represent rotations of 15°» 20° and 25°• It is clear from this

figure and the pole-density diagram (fig. 34) that those joints which

paralleled the axis of rotation underwent the greatest rotation, and

that those normal to the middle-limb trend underwent the least rotation.

Furthermore, given the 20°E rotation, the west half of figure 34 would

represent the structurally highest portion of the fold, the upper limb;

all the joint surfaces dip toward the upper limb. Comparison of figure 68

Figure 34* Pole-Density Diagram Generated by Rotating Selectively Defined Joint Sets 20° to the East about a North-South Axis.

Contour intervals represent 2.5, 5, 7*5, and 10$ intervals. g

25°of rotation

20°of rotation

I50of rotation

Angle between pre-rotation strike of joint set and axis of rotation

Figure 35. Graph Showing the Number of Degrees of Change in Dip for a Joint Set Versus the Angle between the Pre-rotation Strike of the Joint Set and the Axis of Rotation about which the Strata Containing the Joint were Rotated.

Note that those joint sets whose strike approaches that of the axis of rotation undergo the greatest rotations. The three lines represent rotations of 15°, 20° and 25°.

Ox xO 70

34 with the pole-density diagrams for joints and faults measured in the middle limb zones show the same basic pattern.

The second test was designed to compare the attitudes that all fractures would have had prior to monoclonal folding. Fractures meas­ ured in the upper and lower limb strata are considered to be very close to their pre-folding attitudes. In contrast, the fractures measured in middle limb strata were returned to their original attitudes by stereo- graphically rotating poles to joints by the same number of degrees re­ quired to return bedding to the horizontal. The results are shown in figure 36. The open circles represent the poles to the dominant frac­ ture sets measured in the upper and lower limb strata; the x*s repre­ sent the attitudes that the dominant fracture sets, measured in middle limb strata, would have had prior to monoclinal folding. The close groupings of these poles (fig. 36) suggests that the fractures formed prior to monoclinal folding.

Some additional fractures may have been created during mono— olinal folding. The local strain resulting from the extension of the middle limb during draping would favor the formation of a conjugate set of fractures such as is shown in figure 37» However, the attitudes of these fractures would be so similar to .that of the pre-existing sets that it is impossible to distinguish them from the pre-existing fractures.

Kinematics of Distributed Shear

During monoclinal folding stresses were accommodated, in part, by movement along macroscopically spaced concordant and discordant 71

4

x-poles to joints collected from undeformed strata o-poles to joints collected from folded strata

Figure 36. Lower-Hemisphere Projection of Poles that Represent Joint Maxima Measured in Horizontal (x*s) apd Folded Strata (o’s).

The open circles represent the attitudes that the poles to joint maxima would have if the strata containing them were rotated to a horizontal position. th

Figure 37. Strain Ellipse Resulting from the Shear Couple Set up by the Draping of the Sedimentary Cover Rocks during Monoclinal Folding.

Note how the stress field favors the initiation of high-angle reverse faults (after Hills, 1964). 73

shear surfaces. Some movement took place parallel to bedding planes,

as evidenced by slickensides on concordant faults that plunge parallel or sub-parallel to the dip of bedding. Extension of the layered rocks was achieved predominantly by dilation of preexisting joints, which al­ lowed individual blocks to separate and rotate. This dilation created voids that were filled by calcite and slickensided by continued defor­ mation. Because of the structural gradient created during uplift, the individual blocks were down-dropped in stair-step fashion with respect to each other (fig. 38). Blocks closer to the lower limb were down- dropped with respect to the adjacent block up-dip. The overall result of movements along the discordant and concordant fault surfaces was the

extension of the middle limb units in a manner that may be visualized

as the first stages of a brittle mode of boudinage.

The utilization of numerous preexisting fractures as shear sur­

faces can explain the lack of major offsets on these surfaces. The

amount of offset on each of these shear surfaces is constrained by the

number of these surfaces along which movement could take place, and by

the structural relief created during uplift. Assume, for example, that

the displacement to be accommodated across a given zone of weakness is

100 meters (320 feet). If slip occurred along one fault the displace­

ment would be 100 meters (320 feet) per fault. However, if ten faults

were utilized to accommodate displacement, the average displacement per

fault would drop to 10 meters (32 feet); for 100 faults, the average

displacement drops to 1 meter (3.2 feet) per fault; for 10,000 faults,

displacement becomes 0.01 meters (0.03 foot) per fault. Thus, increased 74

I

i i

Figure 38. Schematic Diagram Showing the Inferred Change of Attitude of Pre-folding Joints during Folding, and the Relative Movement of Individual Blocks Bounded by these Surfaces. 75 availability of shear surfaces would distribute the stress throughout the displacement zone.

In the case of the monoclines, displacement per fault is a function of the structural relief across the monocline divided by the number of fractures within the width of the middle limb zone. Because joints pervaded all rock units prior to monoclonal folding, the high number of available surfaces allowed the magnitude of displacement per surface to remain low.

Consideration of the distribution of stress throughout the en­ tire monoclonal fold also helps to explain the lack of large-scale ten- sional faults. In the sedimentary rocks the widening of the middle limb zone and the decrease in dip of this zone higher in the sedimentary section would serve to decrease tensional stress in these rocks (fig.

39). If the vertical component of uplift, b, is constant, and the angle of dip of the middle limb, q , decreases as the fold is translated up­ wards through the sedimentary section, then it follows that a, the orig­ inal length of the unit, must approach c, the extended length of the AS. unit. As a approaches c, the strain, = approaches zero, as does o the extensional stress. As a numerical example; if in figure 39 a = 30 units, b = 25 units and c = 40 units, the extensional strain,

equals or 33.3^. But if a = 65 units, b = 25 units and c = 70 units, the extensional strain now equals or 7.6$. The extensional

strain has decreased by 25.7 percentage points higher in the section.

Because the extension is a function of the amount of vertical

displacement of the basement blocks and the width of the middle limb

zone, it seems probable that the stress in the sedimentary rocks 76

Figure 39• Geometric Diagram Showing how the Increase of the Width of the Middle Limb Zone and the Decrease in Dip of that Zone would Result in a Decrease in the Amount of Extensional Strain Higher in the Fold. 77 adjacent to the basement-cover interface would be so great that it would result in the formation of large-scale grabens in those units. As the middle limb zone widens, the stress decreases to a point where it can be accommodated by thinning of the incompetent units and discrete amounts of slip along the pre-existing fractures. At this point the tensional faults in the sedimentary rocks would be transformed into secondary drape folds.

This process by which faults pass into folds in the middle limb zones might explain the presence of the small anticlines and synclines associated with the upper hinge of the Cow Springs-Organ Rock monocline.

Likewise, the southeast plunging anticline at the Kayenta salient is the surface expression of the "comer" of the Tsegi block (fig. 31b). CONCLUSIONS

Geophysical and geologic data from many parts of the Colorado

Plateau tectonic province indicate that most of the monoclines in the

Colorado Plateau are situated above zones of weakness in the crystalline basement. Where the deep structure of these monoclines is exposed, they are seen to pass with depth into high-angle reverse faults. These faults appear to have undergone multiple periods of movement beginning in the Pre Cambrian.

In the study area residual aeromagnetic and gravity data indi­ cate discontinuities in the basement rocks beneath and parallel to the

Cow Springs, Organ Rock, and Comb Ridge monoclines. These data, cou­ pled with the abrupt monoclinal flexures in the surficial rocks, indi­ cate that the monoclines are the surface expression of high-angle basement faults. The deep structure of the study area is visualized to be dominated by three fault-bounded blocks, the Tsegi, Monument, and Black Mesa blocks. The Tsegi block is structurally the highest block, whereas the Black Mesa block is the structurally lowest block.

Through analogy with other monoclines in other portions of the Colorado

Plateau, and by the determination of the axes and sense of rotation of the associated blocks, the basement faults are inferred to be high- angle reverse faults.

Because of the steepness of these faults, only a small component of crustal shortening would be necessary to produce a large component

78 79 of vertical uplift* This vertical uplift resulted in the passive drap­ ing of the sedimentary rocks over the edges’" of the basement blocks as monocline], folds. The sedimentary rocks accommodated the extensional stress induced during monoclinal folding in two ways. In the incompe­ tent units, extension was accommodated by flexural flow as evidenced by bedding plane cleavage and thinning of units in the middle limb zone.

The competent units can be shown to have been cut by a pre-existing joint pattern, which was utilized to accommodate extension. This pro­ cess was accomplished by the dilation of favorably oriented joints and the separation and minor rotation of individual blocks. REFERENCES

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Shoemaker, E. M., 1956, Structural features of the central Colorado Plateau and their relation to uranium deposits, in Page, R. L. ed., Contributions to the geology of uranium and thorium: U. S. Geol. Soc. Prof. Paper 300, p. 155-170.

Shoemaker, E. M., Abrams, M. J., and Squires, R. L., 1974, The Bright Angel and Mesa Butte Fault systems of northern Arizona, in Eastwood, R. L. and others eds., Geology of Northern Arizona, Pfc. I— Regional Studies: Northern Arizona University, Box 6044, Flagstaff, Arizona.

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16 56 ?. PUBLISHED BY THE OFFICE OF ARID LANDS STUDIES AND THE DEPARTMENT OF GEOSCIENCES UNIVERSITY OF ARIZONA. TUCSON. ARIZONA F O L D S M A P IN CO OPERATIO N WITH COLORADO PLATEAU PROVINCE, ARIZONA THE ARIZONA OIL AND GAS CONSERVATION COMMISSION. PHOENIX, ARIZONA

109( 28 29 R30E ---37 R5W 4 ___ 3 RIE 8 I * y/ ~ 3 ? * | yZ ...... / ' C " i o \ z„ TSfiUSH X X % i 8 ^ z \ > *rl m z \ m m ■ E x A , 2 < x \ A 4?'s ' < 40 V »Xl \ i* of /X X »■/ ^9/ 7 ? • w v 1 /,s s> 7 y i 1/ •V V < u VA .' ■ l i p x y'2- M 38 3 1 \ l F h -a l z t \ 2 , - K ■ \ > V i I \ i f . 6% \V X % i v» V c .^ o \ ft > r f 5 1 % 13 1 1 \ X a / \ . —* . h t — j — * ■ • f— % / X ' S b \% NAVAJO/ HOPI X rJ^ 2- x I y •/ \ \ V X # x L m W v^idge_ : > V\ 9 3\Many X a d r \ t x % yf j p a > y

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j A .J A l \: XX ^SilSkfel^Lvl y \ O m # x 3 "zp- ■- >«r.T v \' x 1 X x ' x # i i ’\ y j i r % o £ / A A i - i s r a e c x •- k .P y i - m -%X # h X XI 8 s r ^ i \ , j 2 \ 3 L/C k w fflp yM i f ' x M 71*1 8 9 ,\ t^SK* - i f x X X m m x x i 1( m s f j t z N1 / . / • - m# x X 7 •• &#E x

7'^Wi ^ I f L F -8 V k m h 1 J i P " | 5F > 2 . TttZ 20 2 \ L -OitJo J T < 4 YM 0*JrJk' a ^'::%JX S3 Xz % ■ n m l! > < f f f V X / # 1 m m m ' \ \ \ ? / % ______> . r h r / i \ i r a i \mx\ - - T - 4 1 ■A - X" J l | S E r , IE A 7 SOURCES OF DATA # ,* _ X Structure mapping of the monoclines was carried out by Davis and Kiven in the summer of 1974. The r-. r -r / open-fold pattern of anticlines and synclines shown on this map, and some monoclinal-fold data, are from previous workers, including: A P # i r I m e Akers, J. P., 1964, Geology and ground water in the central part of — 1967, Preliminary geologic map of the Bright Angel quadrangle. Apache County, Arizona: U. S. Geol. Survey Water-Supply Grand Canyon National Park, Arizona: Grand Canyon Nat. H r Paper 1 771. History Assoc., Grand Canyon, Ariz. _ Akers, J. P„ Irwin, J. H., Stevens, P. R., and McClymonds, N. E., — 1967, Preliminary geologic map of the Grand Canyon and w m W f 1962, Geology of the Cameron quadrangle, Arizona: vicinity, Arizona - eastern section: Grand Canyon Nat. History U. S. Geol. Survey Geol. Quad. Map GO-162 Assoc., Grand Canyon, Ariz. p H Arizona Bureau of Mines, 1958, Geologic map of Yavapai County, -1969, Preliminary geologic map of the Grand Canyon and W w m Arizona: Tucson, Ariz, vicinity, Arizona: Grand Canyon Nat. History Assoc. m Babenroth, D. L., and Strahler, A. N., 1945, Geomorphology and structure Grand Canyon, Ariz. WiT' . of the East Kaibab Monocline, Arizona and Utah: Geol. Soc. McKay, E. J., 1977. Geologic map of the Show Low quadrangle, Navajo America Bull., v. 56, p. 107-150. County, Arizona: U- S. Geol. Survey Geol. Quad. Map GO-973. < 6 % Barnes, C. W., 1974, Interference and gravity tectonics in the Gray M oore, R. T., 19 6 8 , Mineral deposits of the Fort Apache Indian Reservation, JM' Mountain area, Arizona, in Eastwood, R. L., and others, eds.. Arizona: Ariz. Bur. Mines Bull. 177. 1 iAt // / r , i j Geology of northern Arizona, Part 11 -Area studies and field Moore, R. T., Wilson, E. O., and O'Haire, R. T., 1960, Geologic map of r > I guides: Northern Ariz. Univ., Box 6044, Flagstaff, Ariz,, Coconino County, Arizona: Ariz. Bur. Mines, Tucson, Ariz. 'yU /i p. 4 4 2 4 5 3 . O'Sullivan, R. 8 ., »nd Beikman, H. M., 1963, Geology, structure, and ■M Beaumont, E. C., and Dixon, G. H., 1965, Geology of the Kayenta and uranium deposits of the Shiprock quadrangle, New Mexico and WM~ V «C Chilchinbito quadrangles, Navajo County, Arizona: U. S. Geol. Arizona: U. S. Geol. Survey Misc. Geol. Inv. Map 1-345. W(7 Survey Bull. 1202-A. Peirce, H. W., Keith, S. B., and Wilt, J. C., 1970, Coal, oil, natural gas, C o o ley , M. E., Harshbarger, J. W., A kers, J. P., and Hardt, W. F., ^ ■ helium, and uranium in Arizona: Ariz. Bur. Mines Bull. 182. 1969, Regional hydrogeology of the Navajo and Hopi Indian Phoenix, D. A., 1963, Geology of the Lees Ferry area, Coconino m '3 g \ Reservations, Arizona, New Mexico, and Utah: U. S. Geol. County, Arizona: U. S. Geol. Survey Bull. 1137. x \ Survey Prof. Paper 521-A. x 3 ^ \ Scurlock, J. R., 1967, Geologic structure map of northeastern Arizona: Jr 44 D oeringsfeld, W. W., A m u e d o , C. L., and Ivey, J. B., 1 9 5 8 , G en eralized Ariz. Oil and Gas Conserv. Comm., Phoenix, Ariz. ' ^ \ tectonic map of the Black Mesa basin, showing major structural Shoemaker, E. M., Squires, R. L., and Abrams, M. J., 1974, The Bright fm . \/ features, in Guidebook of the Black Mesa Basin, northeastern Angel and Mesa Butte fault systems of northern Arizona, in m i r - fM- u Arizona: N. Mex. Geol. Soc., 9th Field Conf., p. 145. Eastwood, R. L., and others, eds.. Geology of northern I S f f i m Finnell, T. L., 1966a, Geologic map of the Cibecue quadrangle, Arizona, Part I-Regional studies: Northern Ariz. Univ., Navajo County, Arizona: U. S. Geol. Survey Geol. Quad. Box 6044, Flagstaff, Ariz., p. 355-391. A QBteB Map GO-545. Strahler, A. N., 1948, Geomorphology and structure of the West 11 ------1966b, Geologic map of the Chediski Peak quadrangle, Kaibab fault zone and Kaibab Plateau, Arizona: Geol. Soc. M W\ Navajo County, Arizona: U. S. Geol. Survey Geol. Quad. America Bull., v. 59, p. 513-540. Map GO-544. Wanek, A. A., and Stephens, J. G., 1953, Reconnaissance geologic map L 3 # 7 Gregory, H. E., 1917, Geology of the Navajo Country-a reconnaissance of the Kaibito and Moenkopi Plateaus and parts of the Painted ¥4 * of parts of Arizona, New Mexico, and Utah: U. S. Geol. Desert, Coconino County, Arizona: U. S. Geol. Survey Oil m F 3 Survey Prof. Paper 93. and Gas Inv. Map OM 145. & Irwin, J. H., Akers, J. P.. and Cooley, M. E., 1962, Geology of the Leupp Wells, J. D., 1960, Stratigraphy and structure of the House Rock ‘dm m quadrangle, Arizona: U. S. Geol. Survey Misc. Geol. Inv. Valley area Coconino County, Arizona: U. S. Geol. Survey Bull. A F P h e M ap 1-352. 1081-D . Kelley, V. C., 1955, Regional tectonics of the Colorado Plateau and m Wilson, E. D., and Moore, R. T., 1959, Geologic map of Mohave County, % E relationship to the origin and distribution of uranium: Univ. Arizona: Ariz. Bur. Mines, Tucson, Ariz. — ,L& 1 -dih V 3 New Mex. Geol. Pub. 5. Wilson, E. D., Moore, R. T., and Peirce, H. W., 1959, Geologic map of ftmSsSEi ------1958, Tectonics of the Black Mesa region of Arizona, in Gila County, Arizona: Ariz. Bur. Mines, Tucson, Ariz. Guidebook of the Black Mesa basin, northeastern Arizona: Wilson, E. D., Moore, R. T., and O'Haire, R. T., 1960, Geologic map of N. Mex. Geol. Soc., 9th Field Conf., p. 137-144. Navajo and Apache Counties, Arizona: Ariz. Bur. Mines, Tucson, -1967, Tectonics of the Zuni-Defiance region. New Mexico Ariz. and Arizona, Guidebook of the Defiance-Zuni-Mt. Taylor . M in Wilson, E. D., Moore, R. T., and Cooper, J. R., 1969, Geologic map ■ x Hr- ,5^ region, Arizona and New Mexico: N. Mex. Geol. Soc., 'si. ■ T- > of Arizona: Ariz. Bur. Mines and U. S. Geol. Survey. 55 i f * . 18th Field Conf., p. 28-31. Witkind, I. J., and Thaden, R. E., 1956, Preliminary geologic map Krieger, M. H., 1967a, Reconnaissance geologic map of the Picacho of the Dinnehotso SE quadrangle, Arizona: U. S. Geol. Butte quadrangle, Yavapai and Coconino Counties, Arizona: Survey Mineral Inv. Field Studies Map MF-94. ' •I < U. S. Geol. Survey Misc. Geol. Inv. Map 1-500. ------1963, Geology and uranium-vanadium deposits of the M onum ent 1 MSSmiri ^ M\ ------1967b, Reconnaissance geologic map of the Ash Fork quadrangle, Valley area Apache and Navajo Counties, Arizona: U. S. Geol. Yavapai and Coconino Counties, Arizona: U. S. Geol. Survey Survey Bull. 1 1 0 3 . Misc. Geol. Inv. Map 1-499. Witkind, I. J., Thaden, R. E., Johnson, D. H., Finnell, T. L., and Claus, A Lucchitta, Ivo, 1974, Structural evolution of northwest Arizona and its R. J., 1956, Preliminary geologic map of the Dinnehotso ■FH relation to adjacent Basin and Range province structures, in SW quadrangle, Arizona: U. S. Geol. Survey Mineral Inv. Field v # - % , Eastwood, R. L., and others, eds.. Geology of northern Arizona, Studies Map MF 95. Part I -Regional studies: Northern Ariz. Univ., Box 6044, Witkind, I. J., Thaden, R. E., Johnson, D. H., Finnell, T. L., and McKay, w a r " Ms T Flagstaff, Ariz., p. 336-354. E. J., 1956, Preliminary geologic map of the Dinnehotso NW W&,: Maxson, J. H., 1961, Geologic map of the Bright Angel quadrangle. quadrangle, Arizona-Utah: U. S. Geol. Survey Mineral Inv. Grand Canyon National Park, Arizona: Grand Canyon Nat Field Studies Map MF 92. W J 1/ *• History Assoc., Grand Canyon, Ariz. v: m / Sr J& k M s a e s s T ' H _ y L w *4 f 4 i 29 V M s A -; 13 z V X > fy■ ® p 8 ■a .sA r-xkM - J i M \sy / . 2 7 T - Xy S p ,<&CL U A L F nr i ; 8 11 EXPLANATION Strike and dip of beds W- K Strike of vertical beds mil V \ K l p P " ■ Anticline 2 0 < J r Showing trace o f axial plane and direction o f R T f i X'i i ------plunge * 8 -atlf Syncline m a m :. 4------Showing trace o f axial plane and direction o f plunge * X' w f o m m . m -' Monocline tYk, rf Showing trace o f axial plane; arrows point in ■Ej direction of dip of middle limb * mm 3 Monocline m Showing trace o f upper hinge; short arrow on t m * . the relatively steeply dipping middle limb of fold*

Monocline SCALE Showing trace o f lower hinge; short arrow on 1 :5 0 0 ,0 0 0 the relatively steeply dipping middle limb of fold* MAP LOCATION 40 Statute Miles * Dashed where approximately located H H H □

50 Kilometers H H H H hT 1 centimeter equals 5 kilometers STRUCTURE MAP OF FOLDS IN PHANEROZOIC ROCKS COLORADO PLATEAU TECTONIC PROVINCE OF ARIZONA by ACKNOWLEDGMENT The production of this map was funded jointly by Office of University Affairs, National Aeronautics and Space Administration Grant, NGL03-002-313; Arizona Oil and Gas Conservation Commission; and Department of Geosciences, George H. Davis and Charles W Kiven University of Arizona. The satellite image base was provided by the courtesy of Arizona Resources Information System. BASE MAP-MATERIAL PREPARED BY THE U. S. GEOLOGICAL SURVEY Production of the map was aided by the interpretation of regional fold IN COOPERATION WITH THE NATIONAL AERONAUTICS AND SPACE structures from ERTS-1 imagery of northeastern Arizona. This map may be ADMINISTRATION (ERTS-1, PROPOSAL SR 211) accompanied by Bulletin 9, Office of Arid Lands Studies, University of 1927 North American Datum Arizona, entitled "Tectonic Analysis of Folds in the Colorado Plateau of This area also covered by corresponding 1:500,000 geologic 1975 Arizona," which provides additional interpretive information regarding the map of Arizona prepared cooperatively by The Arizona Bureau structural geology of northeastern Arizona and its relation to known oil, of Mines and the United States Geological Survey Figure 2 gas, and uranium deposits. CARTOGRAPHY BY CARTOGRAPHIC ILLUSTRATORS Charles W. Kiven , M S. Thesis, 1976 PHOENIX. ARIZONA Department of Geosciences EXPLANATION r~ >» o c Qal Qd Qt VOLCANIC ROCKS a> E Dark red brown to maroon sandy mudstone and Dikes and plugs of greyish brown augite minette. o

Kmv /< CONTACT MESA VERDE GROUP Dashed where approximate; queried where inferred; Coal-bearing yellow, buff or brown sandstone; dark- dotted where concealed. brown or purplish brown cloystone, siltstone and shale; includes Yale Point Sandstone, Wepo Formation and v> 3 Toreva Sandstone. O Kms O MANCOS SHALE #) CL Light grey, dark grey and ton cloy shale, silty shale , cx 3 0 and very fine-grained sandstone. Z) MONOCLINE Showing trace and plunge of upper hinge; short arrow on relatively steeply dipping middle limb; Kd "a " indicates that hinge is coincident with an anti

DAKOTA SANDSTONE dine, dashed where approximate; queried where Grey to yellowish brown and brown sandstone with inferred; dotted where concealed. medial carbonaceous shale.

UNCONFORMITY

MONOCLINE Kmr Showing trace and plunge of lower hinge; short arrow on relatively steeply dipping middle limb; V) 3 MORRISON FORMATION "S" indicates that hinge is coincident with a syn­ O siltstone or shale of West water Canyon member; pole O reddish brown to greyish pink sandstone and conglo­ a> merate with dark reddish brown to greenish grey $ o mudstone of Recapture member; and light-grey to u greyish brown sandstone with greenish grey and dark- ANTICLINE red brown mudstone of Salt Wash member. oM Showing trace of axial plane and direction of O V) plunge; dashed where approximate,queried where

U V) * -Jn (/) o 3 NAVAJO SANDSTONE "3 SYNCLINE Massive reddish orange to greyish orange aeolion Showing trace of axial plane and direction of cross - bedded, cliff- forming sandstone. plunge; dashed where approximate, queried where inferred; dotted where concealed. •& k i Bed roc k Geology by : KAYENTA SANDSTONE 1) Beaumont, E C.,and Dixon, G H ,(1965) M N 14 1/2 E Pole reddish brown to pale purple sandstone with 2) Wit kind , I. J. , and Thadd en , R. E., (196 3) lesser amounts of clay shale.

UNCONFORM ITY

"R w 36°45

WINGATE SANDSTONE Reddish brown cliff - forming, oeo I ion cross

Tic h 62,000 CHINLE FORMATION Contour interval = 200 feet Grey-green, brown and purple - red varigated clay stone, siltstone and very fine-grained sandstone with dark brown basal conglomerate.

UNCON FORMITY 110° 22' 30'

Figure I. Structure - Geologic Map of the Southwest Corner of the Monument Uplift Charles W. Kiven, M S. Thesis, 1976 Department of Geosciences